S IMPLE S TA R T /S TOP M OTOR C IRCUIT

S IMPLE S TA R T /S TOP M OTOR C IRCUIT

Figure 11-25 shows the wiring diagram for a three-phase motor and its corresponding three-wire control circuit, where the auxiliary contacts of the starter seal the start push button. To convert this circuit into a PLC program, first determine which control devices will be part of the PLC I/O system; these are the circled items in Figure 11-26. In this circuit, the start and stop push buttons (inputs) and the starter coil (output) will be part of the PLC system. The starter coil’s auxiliary contacts will not be part of the system because an internal will be used to seal the coil, resulting in less wiring and fewer

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S ECTION PLC System Programming C HAPTER 3 Programming

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Push Button

Station

OL

(three-wire control)

Figure 11-25. (a) Wiring diagram and (b) relay control circuit for a three-phase motor.

Figure 11-26. Real inputs and outputs to the PLC.

connections. Table 11-11 shows the I/O address assignment, which uses the same addressing scheme as the circuit diagram (i.e., inputs: addresses 000 and 001, output: address 030).

To program the PLC, the devices must be programmed in the same logic sequence as they are in the hardwired circuit (see Figure 11-27). Therefore, the stop push button will be programmed as an examine-ON instruction

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S ECTION PLC System Programming C HAPTER 3 Programming

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Table 11-11. I/O address assignment.

Stop Start

Figure 11-27. PLC implementation of the circuit in Figure 11-25.

(a normally open PLC contact) in series with the start push button, which is also programmed as an examine-ON instruction. This circuit will drive output 030, which controls the starter. If the start push button is pressed, output 030 will turn ON, sealing the start push button and turning the motor ON through the starter. If the stop push button is pressed, the motor will turn OFF. Note that the stop push button is wired as normally closed to the input module. Also, the starter coil’s overloads are wired in series with the coil.

In a PLC wiring diagram, the PLC is connected to power lines L1 and L2 (see Figure 11-28). The field inputs are connected to L1 on one side and to the module on the other. The common, or return, connection from the input module goes to L2. The output module receives its power for switching the load from L1. Output terminal 030 is connected in series with the starter coil and its overloads, which go to L2. The output module also directly connects to L2 for proper operation. Note that, in the motor control circuit’s wiring diagram (see Figure 11-29), the PLC output module is wired directly to the starter coil.

Although the three-phase motor has a three-wire control circuit, its corre- sponding PLC control circuit has only two wires. This two-wire configuration is similar to a three-wire configuration because it provides low-voltage release; however, it does not provide low-voltage protection. Referring to

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S ECTION PLC System Programming C HAPTER 3 Programming

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M Common

Figure 11-28. PLC wiring diagram of a three-phase motor.

L1

L2 L3

PLC Output 030

Figure 11-29. Motor control circuit’s wiring diagram.

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S ECTION PLC System Programming C HAPTER 3 Programming

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Figure 11-29, the starter’s seal-in contacts (labeled as 3—| |—2) are not used and are shown as unconnected. If the motor is running and the overloads open, the motor will stop, but the circuit will still be ON. Once the overloads cool off and the overload contacts close, the motor will start again immedi- ately. Depending on the application, this situation may not be desirable. For example, someone may be troubleshooting the motor stoppage and the motor may suddenly restart. Making the auxiliary contact an input and using its address to seal the start push button can avoid this situation by making the two-wire circuit act as a three-wire circuit (see Figure 11-30). In this configuration, if the overloads open while the motor is running, the coil will turn off and their auxiliary contacts will break the circuit in the PLC.

Stop Start

Figure 11-30. Two-wire circuit configured as a three-wire circuit.